This invention concerns underdrain filtration systems, especially sand filters wherein metal underdrain units extend longitudinally at the bottom of the medium. More specifically it is concerned with backflushing of the underdrain units and surrounding medium to remove particulates or cake on the outside of the underdrain unit and in the surrounding filter medium.
Current stainless steel underdrain products are essentially limited to one of two configurations: (1) an air scour lateral is internal but positioned at the top of the underdrain compartment; or (2) the air laterals are “outside”, separate from the underdrain unit, either a pipe or rectangular tube (such as shown in U.S. Pat. No. 6,090,284). Fine openings that make the underdrain unit “media retaining” are typically formed in separate panel pieces bolted or welded to the outside of the underdrain body, although in some underdrain systems the small openings were slits formed in the folded metal underdrain structure itself. There are currently three products fitting the above description. Two current stainless steel underdrain products can have the air feed from the gullet, from below the underdrain; one cannot. Two of the current stainless steel products do not scour the bottom four inches of filter media in the filter with air. The air scour chamber and ports are located too high in the product to allow full depth scouring.
The following U.S. patents and publications have some relevance to the invention: U.S. Pat. Nos. 2,710,692, 4,065,391, 4,214,992, 4,322,299, 5,019,259, 5,269,920, 5,328,608, 5,639,384, 6,090,084, 6,325,931, 6,569,328, 6,691,413, 6,797,166, 2002/0096463, 2004/007541, 2008/0245750 and 2014/0021121.
Prior underdrain units have not exhibited the efficiency and overall effectiveness in cleaning the exterior underdrain units and surrounding filter medium as in the current invention.